Stencils and screens to apply patterns or printable material on substrates have been used in many contexts. For example, such a patterned adhesive layer, as described in Farnworth et al., U.S. Pat. No. 5,286,697, which is hereby incorporated by reference, may be used to attach a semiconductor die to a lead frame. Farnworth et al. describes the use of a patterned screen to deposit an adhesive layer on the die so that the die can be mounted to the lead frame. The screen is patterned so that the streets between individual dies on a wafer are free from adhesive to permit later sawing of the wafer into separate the dies, and so that the bond pads are also free from adhesive to permit later connection of the bond wires between the lead fingers of the lead frame and the bond pads.
Farnworth et al. specifically teaches a screen printing process in which the patterned screen is situated between the wafer and a liquid adhesive nozzle. Once the screen is properly aligned over the wafer, liquid adhesive is released from the nozzle, coating the wafer with adhesive in the desired pattern. The screen is then removed from the wafer, and the dies of the wafer are attached to a lead frame and separated from one another. The application of liquid adhesive on a wafer using a screen as described in Farnworth et al. is similar to a silk screen process used for printing artwork, such as on T-shirts. This same stencil and screen printing process similar to the silk screen process used for printing artwork on T-shirts has also been utilized in other semiconductor applications. Such semiconductor applications include printing a patterned nonconductive polyimide barrier for flip chips, printing conductive adhesive bumps or solder bumps, and printing conductive ink on flexible printed circuit boards (PCBs).
However, it has been found that a shortcoming to such screen and stencil printings is the migration or overflow of the printable material (such as the adhesive or the ink) underneath the bottom of the screen, which causes the substrate to which the printable material is being applied (such as the wafer or the printed circuit board) to stick to the screen. This results in a poorly patterned layer. More significantly, migration underneath the bottom of the screen shortens screen life, which is defined as the number of consecutive uses, or prints, of the screen without cleaning of the screen. Migration underneath the bottom of the screen forces cleaning of the screen between prints to ensure consecutive clear prints. Reduced screen life therefore slows down the screen and stencil printing process, and adds cost to the process.